How do you make flat shading for games?
Shaders are managed by the render engines. If you want to make assets for a video game, then the render engine is the game engine, and so the way to make shaders in it depends on him, not on Blender. The shaders will be made in your game engine and might not have an equivalent in Blender.

The only thing you can do in Blender is to prepare the textures you will use in your shaders, as well as assign materials slots on your mesh if you need several materials on a single mesh.